1. Field of the Invention
The present invention concerns an optical transmission process and system for sending solitons over very long distances.
2. Description of the Prior Art
Light pulses can now be transmitted in optical fibers over intercontinental distances due to the development of optical amplifiers such as doped fiber and semiconductor amplifiers, Raman amplifiers, and parametric amplifiers. These amplifiers enable data transmission at much higher bit rates and at much lower cost than electronic regenerators. FIG. 1 is a diagrammatic representation of a transmission system between a transmitter sender (E) 2 and a receiver (R) 8 comprising optical amplifiers (AO) 6. The system comprises multiple sections (T.sub.f) 4 of monomode optical fiber interconnected to transmit light pulses. The system also includes devices which amplify these light pulses by injecting optical energy into them without converting them into electronic pulses. The optical amplification may be "distributed" all along the fiber (as in the case of Raman amplifiers and low doped fiber amplifiers) or "lumped" at relatively compact modules spaced by 25 to 150 km apart depending on the characteristics of the transmission system.
However, optical amplification does not re-shape the transmitted signal. Thus, the distortion of each pulse, as it propagates, is cumulative, restricting the reliability of transmission. Optical pulses are usually affected by two physical phenomena causing distortion: chromatic dispersion and the optical non-linearity of the fiber.
There are, however, special light pulses called "solitons" whose shape and intensity characteristics are such that the two types of distortion (chromatic and nonlinear) cancel out. This cancellation occurs when the chromatic dispersion of the fiber is "anomalous" with a negative coefficient for the group velocity dispersion. For the silica fibers used presently this condition is satisfied in the so-called minimal attenuation" window in the vicinity of the wavelength of 1.5 .mu.m.
The propagation of light pulses in optical fibers is expressed by the non-linear Schro/ dinger equation. Light pulses in which the profile of the electric field is the shape of a hyperbolic secant (sech.times.=1/cosh .times.) are specific solutions of this equation and constitute solitons. These optical solitons can therefore propagate in an optical fiber over intercontinental distances with no distortion provided that their attenuation is compensated by periodic amplification. The current state of the art and performance characteristics of optical communication systems using soliton pulses are summarized in the following reference documents:
1! S. G. Evangelides, L. F. Mollenauer, J. P. Gordon, and N. S. Bergano, "Polarization multiplexing with solitons", Journal of Lightwave Technology, vol. 10, 28-35 (1992).
2! M. Nakazawa, K. Suzuki, Eo Yamada, and H. Kubota, "Observation of nonlinear interactions in 20 Gbits/s soliton transmission over 500 km using erbium-doped fiber amplifiers", Electronics Letters, vol. 27, 1662-1663 (1991).
3! L. F. Mollenauer, M. J. Neubelt, S. G. Evangelides, J. P. Gordon, J. R. Simpson, and L. G. Cohen, "Experimental study of soliton transmission over more than 10000 km in dispersion-shifted fiber", Optics Letters, vol. 15, 1203-1205 (1990).
4! J. P. Gordon and H. A. Haus, "Random walk of coherently amplified solitons in optical fiber transmission, Optics Letters, vol. 11, 665-667 (1986).
The prior art has the drawbacks described below.
Amplitude modulation coding as currently used for soliton transmission, is of the all-or-nothing on-off keying (OOK) type. The presence of a pulse in a time window (a time interval defined by a clock and also known as the "bit period") represents the binary digit 1 and its absence denotes the binary digit 0. For example, transmitting the binary digit 11 entails launching two solitons into the fiber in two consecutive bit periods. For each of the two solitons the presence of the other disturbs the delicate equilibrium between chromatic and non-linear distortion and this is manifested as interaction between the solitons: as they propagate the two solitons coalesce and separate periodically. If the initial separation q.sub.o between the solitons is in the order of three soliton widths LS (q.sub.o =3 LS, with the soliton width defined as the full width of the pulse at half-maximum), the pulses coalesce after propagating 450 km which compromises the reliability of decoding beyond 350 km (see reference 2!). Moreover, reliable transmission over 10000 km (for which the coalescence distance is 13000 km) requires an initial separation q.sub.o =6 LS (see reference 3!). Consequently, in addition to restricting transmission range, interaction between solitons sets a minimal value for the bit period, which restricts the bit rate.
A second restriction on the bit rate and range of soliton transmission results from the noise which is introduced upon the optical amplification of the pulse stream. This noise is usually manifested as a random component of the speed of the soliton, causing at the receiving end some uncertainty as to the position of the soliton in the bit period frame. The range and bit rate for reliable transmission are therefore restricted to values for which the jitter of the soliton due to amplification noise does not exceed a fraction of the bit period. This is the Gordon-Haus limit; (see reference 4!).
4!J. P. Gordon and H. A. Haus, "Random walk of coherently amplified solitons in optical fiber transmission", Optics Letters, vol. 11, 665-667 (1986).